Dorchestertown (“parish”parish), West Dorset district, administrative and historic county of Dorset, southwestern England, on the River Frome. Dorchester is the county town (seat) of Dorset.

The ancient town (then known as Durnovaria) was a sizable Roman British centre, and many remains of the period (including mosaics and ruined villas) have been found. In the south an amphitheatre at Maumbury Rings dates from pre-Roman times; Maiden Castle (2 miles [3 km] southwest), a vast earthwork encircled by entrenchments and ramparts and occupying more than 120 acres (50 hectares), was the site of important settlement from Neolithic times into the Iron Age.

As early as the 10th century the town had a mint. By 1086 it was a royal borough, and a castle had been built by the 12th century; the Franciscan priory, founded before 1331, is thought to have been constructed from its ruins. The first charter of incorporation was dated 1610. In 1834 the Tolpuddle martyrs were sentenced in the town for administering illegal oaths concerning trade union activities. The writer Thomas Hardy was born near Dorchester, the “Casterbridge” of his Wessex novels.

The cloth industry flourished in the 16th century, and serge was manufactured in the 17th. The town has been noted for its ale since the 1600s. Dorchester now functions as a market town and serves an extensive rural area. Agricultural machinery, printing, and leatherworking are local specialities. Pop. (2001) 16,171; (2011) 19,060.